Good awareness of the general shot composition (uncluttered backgrounds, good exposure, etc.). Since you have those elements under control, the next step is to help your subjects relax and pose more. In many of these shots they look stiff. A pro model knows how to mix things up a bit, your average cosplayer less so. Compliment them and have fun!

As another small suggestion, take the time to identify the characters in your photo descriptions. It will help people find your photos and makes the shots more than a hallway moment.

Great job oveall, the only thing I noticed is on some of the picture, a tiny parts of the object are cropped (left out from the picture), like the girl's photo sitting down, her skirt is a bit croppe, and the trio picture, the one on the left, her elbow's cropped. Maybe it's just me, but usualy I try to get everything in the frame unless it a close up, or there's a purpose to it.